Create new WorkflowsInternal API for ai_workflows scope

What does this MR do and why?

This MR does not introduce any new API points but instead just moves them from Workflows to WorkflowsInternal. The previous Workflows API contains a mix of endpoints that are consumed by the Duo Workflow Service as well as the VS Code extension in GitLab. The issue is that the Duo Workflow Service only has an ai_workflows scope token which is meant to be limited to only a narrow set of things it can do. The VS Code extension has an api scoped token.

This MR splits the API up such that the endpoints needed by Duo Workflow Service are all inside the WorkflowsInternal API.

Resolves #500057 (closed)

Changelog: other

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Related to #500057 (closed)